Among their scenarios, was the idea that fleets of hi-tech alien probes invisible to our relatively primitive technology could have explored our galaxy by using a 'slingshot' technique. And the odds, suprisingly, aren't against it.

Hello?

The researchers said: "Interstellar probes can carry out slingshot manoeuvres around the stars they visit, gaining a boost in velocity by extracting energy from the star's motion around the Galactic Centre,."

Humanity's Voyager space probe uses this technique, but use planets as the slingshot rather than stars meaning the speeds achieved are not as great.

In order to search the entire Milky Way in 10 million years (not long in space terms) our aliens would have to travel at 1/10 the speed of light.

So why have we not spotted them? It could be because they are too well 'cloaked' or, more likely, they're just not here.

Forgan said: "The fact we haven't seen probes of this type makes it difficult to believe that probe building civilisations have existed in the Milky Way in the last few million years."

The findings once again highlight the 'Fermi Paradox' which postulates: Given that alien technology has had time to develop sufficiently within the timeframe of the universe, why have we not seen it yet?

Or more simply: Where is everybody?

The researchers also drew on other previous work in their findings including a 1983 study by Nasa space expert Robert Freitas.

Forgan said: "The probe camouflages itself so as to set up a threshold test of the technology or intelligence of the recipient species, where the test must be met before the species is allowed to communicate with the device."

Dr. Anders Sandberg, of the Future of Humanity Institute at Oxford University, had a more downbeat explanation.

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Dusty Space Cloud

This image shows the Large Magellanic Cloud galaxy in infrared light as seen by the Herschel Space Observatory, a European Space Agency-led mission with important NASA contributions, and NASA's Spitzer Space Telescope. In the instruments' combined data, this nearby dwarf galaxy looks like a fiery, circular explosion. Rather than fire, however, those ribbons are actually giant ripples of dust spanning tens or hundreds of light-years. Significant fields of star formation are noticeable in the center, just left of center and at right. The brightest center-left region is called 30 Doradus, or the Tarantula Nebula, for its appearance in visible light.